Televisions or videocassette recorders are provided with an on-screen function of displaying information that indicates the state of the device, such as the channel or the programming of recording, on the television screen.
Initially, a conventional on-screen display device will be described with reference to FIGS. 9(a) and 9(b). FIG. 9(a) is a diagram illustrating a structure of the conventional on-screen display device.
In FIG. 9(a), a display character setting unit 100 writes character data that comprise a character code of a desired character to be displayed (hereinafter, referred to as display character) and an attribute code that indicates modification information such as the color of the display character or the background thereof, at a predetermined position in a video RAM 200.
The video RAM 200 has addresses corresponding to respective positions on the display on which the display characters are displayed. FIG. 9(b) shows an address map of the conventional video RAM 200. On one screen of the display, 12 lines of 24 characters can be displayed. In addition, the character data is composed of 9 bits, 8 bits of which are a character code indicating up to 256 types of characters, and 1 bit of which is an attribute code.
A display control unit 300 reads character data from the video RAM 200 before starting to display characters of desired positions and outputs the character data to the buffer 500. In addition, the display control unit 300 reads font data corresponding to the character code included in the character data that is read from the buffer 500, from a character generator ROM 400, and generates an on-screen output signal using the font data.
In the character generator ROM 400, plural kinds of font data are stored.
In the buffer 500, the character data that are read from the video RAM 200 are stored.
The operation of the on-screen display device that is constructed as described above will be described.
First, the display character setting unit 100 writes character data corresponding to one screen, which comprise character codes and attribute codes of characters to be displayed, at addresses in the video RAM 200 corresponding to positions on the display at which the characters are to be displayed, as indicated by the address map of the video RAM in FIG. 9(b).
Then, the display control unit 300 reads the character data that have been written in the video RAM 200 at a predetermined timing, and stores the data in the buffer 500. Thereafter, the display control unit 300 reads font data stored at addresses that are indicated by the character codes included in the character data outputted from the buffer 500, from the character generator ROM 400. Then, the display control unit 300 outputs an on-screen output signal on the basis of the font data read from the character generator ROM 400 and the attribute codes included in the character data that are read from the video RAM 200.
In this way, characters to which desired character modification has been performed can be displayed on the display. In order to further add the character modification (coloring of the display character itself, the color of the background, and the like), details of the character modification are previously stored in the video RAM 200 (see Japanese Published Patent Application No. Hei.9-54575).
In the conventional on-screen display device, however, when the bit length of character data is larger than the bit length of data in the RAM that is accessed, an empty space is generated in an area of the video RAM for holding the character data, and accordingly the video RAM area cannot be used effectively.
Particularly in a case of utilizing the video RAM area also as the RAM area of the CPU (see Japanese Published Patent Application No. Hei.11-102352), the video RAM area would not be used although it is capable of storing data. More specifically, when the bit length of data that is accessed in the RAM of the CPU is 8 bits and the character data is composed of 9 bits as shown in FIG. 9(b), 7 bits in an address for storing the attribute code are not employed.